@JonnyRadu[ch65306]
It's easy to make any frequency of square wave and triangular wave -- of cause in the frequency bands that AVR I/O can support.
But I still want to find a better way to make the sine wave without using table, it's hard to generate the specific frequency in this way.
@liudr
Just build it as a toy, in practical application this R/2R ladder chip will better .. like a standard 5tone generator 